Field of the Invention
This invention relates generally to the stabilisation of starch suspensions and more specifically to the use of stabilised starch suspensions in paper and board manufacture. Stabilised starch suspensions according to the invention can be stored for commercially useful periods of time and can also be pumped and supplied through pipes and ejected through nozzles without offering excessive resistance to such operations, thus enhancing the range of use of starch suspensions.
In the cereals-processing industry it is often the aim to produce protein-enriched products and the concomitant of such procedures is the production of residues in the form of starch suspensions. Hitherto, there have been difficulties in disposing of such suspensions economically. Drying off the suspensions to produce powdered starch is inordinately expensive, given present-day energy costs. In fact, Kirk-Othmer, "Encyclopedia of Chemical Technology", Third Edition, Volume 21 page 499, reports that "The large amounts of energy consumed in evaporating and drying starch make the wet-milling industry the second most energy-intensive food industry in the United States." To the knowledge of the present applicants no satisfactory method of stabilising the starch suspensions, allowing them to be transported and stored without difficulty, has been developed and commercialised. There are clear problems in the way of such a development because any stabilising agent must be such that, whilst maintaining the starch in suspension, it also reacts rheologically in a manner which does not prevent or unduly hinder the suspension being pumped and passed through pipes.
The present invention is concerned, in one aspect, with the problem of stabilising starch suspensions and is thus potentially of great advantage, in the cereals-processing industry in particular.
The invention is also concerned with the use of stabilised starch suspensions in the manufacture of paper and board.
Use of a stabilised starch suspension according to the invention in paper or board making can be especially advantageous if the suspension contains a film-forming polymer which may be a cellulosic material. The stabilised starch suspension of the invention, whether or not containing a film-forming polymer, which may be a cellulosic material, may be added during the early stages of paper or board manufacture or may be added to the pulp itself and may assist web formation during those early stages of manufacture.
Introduction of a stabilised starch suspension according to the invention in the early stages in the formation of a paper web ("wet-end addition"), i.e. when the water content is reduced from about 99% to about 30%, ensures that the suspension is distributed more or less uniformly throughout the thickness of the paper or board. The suspension may also be applied to the formed web, on one or both surfaces thereof, in stages of manufacture subsequent to web formation, as the water content is reduced to between 2% and 20% by weight of the web.
"Size-Press Addition" may also be used to introduce additives in paper or board making, including solutions or dispersions of the additives, or foamed liquids, or dry powders. Whilst the size-press process is a useful method of applying substances to paper and board, large amounts of energy are needed to drive the machinery and to carry out the extra drying operations consequent on the use of a size press and the equipment is expensive to buy and install. Thus, when size-press addition is used to introduce solutions ior dispersions in water, it is necessary to arrange for extra drying equipment to be installed in addition to that used to dry the paper or board before it is passed through the size press. This extra drying requires up to 30% extra drying equipment and represents a further 30% increase in energy consumed during the manufacture of the paper or board.
The present invention which enables stabilised starch suspensions to be applied in a variety of ways and at a variety of stages of paper or board manufacture can be of significant advantage in the paper-making industry.
When dispersions of starch in water are subjected to high rates of shear, e.g. high rates of flow through restricted orifices such as those which are normally used to produce sprays, they exhibit considerable dilatancy, i.e. the apparent viscosity or resistance to flow increases as the rate of shear increases. The result of such increases is to restrict or stop flow. Dilatancy is particularly noticeable in suspensions of starch where the starch represents 5% or more of the total fluid and is familiar to those users of starch who wish to spray suspensions of raw or processed starch as aids to various industrial processes including application to paper and board during and subsequent to manufacture.
Aqueous dispersions of starch also tend to form sediments which are often the cause of failure of equipment. In order to prevent or reduce the occurrence of sedimentation, it has hitherto been customary to increase the viscosity of the suspension by raising the temperature of the starch above the temperatureat which the individual particles rupture; the process is generally referred to as "cooking". This enables the starch to hydrate, forming a gel which, by increasing the fluid viscosity, reduces the tendency of the starch particles to sediment. Alternatively, it is customary to add substances which, by raising the viscosity of the aqueous phase of the suspension, reduce the sedimentation process. Such viscosity-increasing substances include cold-water-soluble derivatives of starch or of cellulose and many high molecular weight polyhydric substances such as polyethylene oxide ethers, polyvinyl alcohols as well as "swelling clays" such as bentonite, attapulgite, "LAPONITE" and the like which are capable of raising the viscosity of the dispersion of starch in cold water.
However, such procedures as described above lead to poor spray formation since the fluids require high pumping pressures in order to achieve a sufficiently high degree of turbulence at the spray head to poduce the fine droplets that are desirable for application as herein described. Such high pumping pressures frequently lead to failure of equipment due to the inherent tendency of starch suspensions to become dilatant. The present invention enables these disadvantages to be overcome.
It is known from U.S. Pat. No. 3,692,552 to prepare stabilised laundry starch by adding powdered starch to an aqueous solution of xanthan gum or a water soluble derivative thereof. But this patent does not address the problem of stabilising an existing starch suspension and particularly of stabilising a starch suspension of the volume and in the condition to be expected of a by-product of cereals processing. Nor does the disclosure touch on the rheological problems of successful stabilisation of industrial starch suspensions.
European Patent Application No. 80300848 (Publication No. 0018 086) discloses the addition of xanthan gum to cooked or modified starch to prevent retrogradation. It is not directed to the problem of stabilisation of starch suspensions or the problem of transport and disposal of by-products of cereals processing.